Influence of elemental B addition on the heat-treated cast structures of Ti-47Al-2Cr-(2-4)Nb alloys

Abstract

The influence of elemental B addition on the heat-treated cast structure of Ti-47Al-2Cr-(2-4)Nb alloys has been investigated using x-ray diffractometry, optical microscopy, scanning and transmission electron microscopy, and tensile testing. The phase sequence is beta --> beta + alpha --> alpha --> alpha + gamma --> alpha + beta + gamma --> beta + gamma. The addition of (0-2 at. %) B does not change the phase sequence. It, however, tends to stabilize a phase by shifting the (alpha + beta + gamma) three-phase region toward a higher (Cr + Nh) content. The B addition does not significantly alter the equilibrium composition within (alpha + gamma) two-phase field. The B addition markedly accelerates the lamellar formation kinetics and enhances the thickening rate of gamma plates, despite the fact that it increases both the misfit between alpha and gamma plates and the alpha/gamma interfacial energy. The acceleration of lamellar formation kinetics is thus believed to be primarily due to the enhancement of chemical diffusivity as a result of B addition. The B addition induces a significant refinement of heat-treated cast structure. This is primarily due to the role of boride to disperse the interdendritic gamma regions to a fine network and to refine the dendrite cell size. Further refinement arises from the boride's role to act as the nucleation site for gamma grains and from the intrinsic B effect to enhance the chemical diffusivity and the gamma thickening rate. The addition of a small amount of B enhances both the strength and tensile ductility of near gamma structure. The strengthening arises from grain size refinement and from the boride dispersion. The calculation of fracture strain suggests that an enhancement of ductility for small B addition (up to similar to 0.2 at. %) is mostly due to its effect to refine the gamma grains.